It's art but it's also a treasure hunt -- a race to locate the work before the vandals. Next to each piece is a toll-free number that the finder can call to receive an audio commentary of the work whilst standing in front of it.

Banksy was an early adopter of social media.

It was the perfect tool to allow him to communicate directly with fans whilst also protecting his anonymity. He's using Instagram because it doesn't matter where you are, you can see what's going on. The more traditional use of a toll-free number is intended to make it feel like you're in a museum.

The lingering question is: Will anyone locate Banksy himself and publish that picture on Instagram? Well I've been trying for years.

Apparently I've even met him without knowing it. But this month I hand the task over to New Yorkers ...